Good day everyone,

I have a question and I didn't manage to find a clear answer so I'm hoping you could offer some opinions.

So I have an S7-1500 system with an RS485 module which I intend to use as a Modbus RTU port. The PLC (and the port, obviously) will be slave to a hierarchically superior system. I'm familiar with Modbus and have configured such systems but I only needed to have holding registers at the disposal of the Master system. This is, however different, as I will have to have holding registers and also coils. Also, some values will need to be written, not only read, both coils and registers.

So, how can I go about this?

Can I have everything in 1 DB?

Is there a sample program I could study?

Can I even do everything with the same port? I didn't really find a clear answer in the documents or on the forum.

Thank you.

Much of what you are asking is documented in the in-software help under the heading "Modbus_Slave: Communicate as Modbus slave". 

The Modbus_Slave instruction links the holding registers with a memory location (DB or M-bits). But it also makes it possible for the Master to point to the inputs and outputs. Be on the look out for a table specific to the S7-1500. (In my version of the help file an A and O is used instead of a Q where referring to output addresses.)

What I don't understand is whether you can have the coils and registers in the same DB. And how would they be pointed by the Master. Should I have the coils first? Do I need different DB's? This is why I asked about a program I could study.

Thank you,

Ovs31

What I don't understand is whether you can have the coils and registers in the same DB. And how would they be pointed by the Master. Should I have the coils first? Do I need different DB's? This is why I asked about a program I could study.

Thank you,

In the document that I have referred  to earlier, contains this information:

You can also refer to "Overview of the Modbus RTU communication" to see the Modbus function codes that are available on the PLC.

Bit read/write actions cannot be done on DB entries. Any bit read/write function is only done to the process image inputs / outputs.

Unfortunately I know of no Siemens example that highlights the use of bit read/write. You are welcome to ask Siemens via the "Support Request" link.

Thank you,

But I cannot find where to select function codes or establish the addresses of the bits. I only have MB_ADDR and MB_HOLD_REG as inputs in my slave block. This only allows me a DB and they are being read as words.. I couldn't find these settings in the instance DB of the Modbus_Comm_Load and my modbus slave function either.   

Am I looking in the wrong place?